URL key is not normalized in `_validate_kas_infos()`

[b-long]

The function _validate_kas_infos() in "packages/otdf-python/src/otdf_python/tdf.py" does not normalize URL keys.

PR #167 made significant improvements to the codebase. The PR was merged without this fix so that we could discuss it in isolation.

Taken from Gemini review:

When deduplicating KAS URLs, comparing them with case sensitivity can lead to duplicate entries if there are minor casing differences in the scheme or hostname (e.g., http:// vs HTTP:// or localhost vs Localhost). Normalizing the URL key to lowercase ensures robust deduplication.